Studies will be conducted to determine if the accumulation of macrophages in ascites tumors is an expression of host resistance to tumor growth. The capacty of animals to produce macrophages will be compromised by lethal irradiation, and growth of peritoneal ascites tumors will be followed in these versus normal animals. Since the macrophage response in ascites tumors may represent an expression of T-cell dependent concomitant anti-tumor immunity, tumor growth will also be followed in T-cell deficient mice. As a direct measure of host resistance, macrophages isolated from the tumors will be tested for their capacity to inhibit growth of tumor cells in vitro. Experiments will also continue to investigate why host cellular defenses fail to be expressed within malignant tissues. Animals immune to Listeria will be tested for their capacity to express immunity against a lethal challenge of bacteria infused into 2, 4 or 6 day old ascites tumors. Failure to express immunity could be caused by: (a) the absence of mediator T-cells from the site of infection, (b) if present, inhibition of the T-cell response to bacteria, or (c) failure of macrophages in the tumor to become activated via contact with T-cells or their products. The first of these possibilities will be explored by determining the number of T-cells present by indirect fluorescence with anti-Thy antibodies. The capacity of mediator T-cells in culture, to liberate lymphokines in the presence and absence of tumor cells and ascites fluid will be tested. The products released by the cells, along with positive controls will be tested for their capacity to activate macrophages from normal and tumor-bearing animals.